Feynman said, "No one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a question of usage, and there is no specific, important physical difference between them."
I've noticed many cases here where people get thrashed, for example, for asking about the relationship between "diffraction and wavelength," because the person doing the thrashing is thinking actually of interference.
It's pretty simple. Diffraction is the spreading out or bending of light-waves upon traveling next to objects or exiting through openings (in the process of at least the latter of which the light-waves also become more spatially-coherent). While interference is the delivery or non-delivery of the light's energy at given points in space in connection with the light-waves' relative phases.
Even if that's not a perfect distinction, still, why such confusion? Is Feynman responsible for this abomination?
Examples of the Confusion here on Physics StacksExchange.
1) Question about diffraction and the relation between slit-width and wavelength.
Member with high reputation comments soon after question is posted, "You can clearly see diffraction (single slit diffraction) with a green laser pointer (so about 0.5 micron wavelength) through a 150 micron slit. That would seem to invalidate your question, unless a factor of 300x is not 'larger'." (This member is speaking of interference.) https://physics.stackexchange.com/questions/253532/diffraction-and-wavelength
2) Another question about diffraction and wavelength relationship.
Another member with high reputation comments soon after question is posted, "Wavelength doesn't affect diffraction at all, especially if you handle all your spatial values in units of wavelength." (This member is speaking of interference.) Diffraction wavelength relationship